Untimely discovery

by Thomas Cooper

How the butterfly got its proboscis

Enlargement: Surrounded by fossilized organic matter (e.g., pollen) is a wing piece that is actually about half the diameter of the period at the end of this sentence. Image: Bas van de Schootbrugge. Click image to enlarge.

In November 2012, Paul Strother, a research professor in the earth and environmental sciences department, was peering through a microscope in Frankfurt, Germany, when he noticed something that set off a five-year scientific investigation and ended up calling into question a long-held understanding within evolutionary biology.

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Strother, a paleobotanist who specializes in “the early evolution of land plants,” was in the office of Bas van de Schootbrugge, a geoscientist at Goethe University. The Boston College professor was examining slides showing bits of core samples taken a year or two earlier at a nature preserve north of the Harz Mountains in Germany. The samples contained fossil remains of life forms some 200 million years old. Many of them were familiar to both scientists: prehistoric spores, pollen grains, plant debris, insect body parts. But, as often happens, there were unrecognizable mysteries. “We call them ‘Mister Greys,'” says Strother.

Strother is primarily interested in the transition between unicellular and multicellular organisms, from aquatic organisms like algae to land plants. That day, he recalls, “I was looking for ancient forms of freshwater algae.” However, as he scrolled methodically along one slide, searching for striations that would indicate a euglena, a diverse group of single-celled microorganisms, his eye caught an unusual striated form.

“My first thought was, wow, maybe I’ve got some euglenas. Then I remembered, I’d seen these things before in a textbook, and I knew right away they were moth scales.” He and van de Schootbrugge compared the ribbed shapes on the one-by-three-inch slide to images in a reference book and decided that Strother had indeed detected the scales of an ancient moth wing. But the scales in the book belonged to moths from a period approximately 60 million years ago, far more recent than the 200-million-year-old scales Strother was looking at. So, what moth could this be?

The two researchers showed images of the scales to lepidopterists in Germany and Chicago and were told the fragments were “not diagnostic,” they lacked tell-tale structural characteristics that would allow them to be dated or in other ways classified with precision. They might belong to ancient moths, or they might be from mosquitoes or flies, which also have scales on their wings. At one point, Strother changed the labels on his images from “moth scales” to “insect scales,” thinking, “We had no proof.”

Fast forward two years to a conference in Paris. Strother, who is tall and lanky with shoulder-length blond hair and a close-cropped beard, was sitting across a dinner table from Torsten Wappler, a palaeoentomologist at Germany’s Hessisches Landesmuseum. Wappler was part of a team that had recently published an exhaustive family tree of insects, going back nearly 500 million years. Strother hauled out his laptop and showed Wappler images of the scales he had seen in Frankfurt, and was told that by making extensive structural comparisons against the new family tree it would be possible to identify the insects shown on the laptop.

Van de Schootbrugge, by then a professor at Utrecht University in the Netherlands, recruited a student to prepare some 70 scale specimens for viewing with a scanning electron microscope (SEM). The work called for an unusual mix of makeshift and high-end technology. To the end of a wooden dissection probe, the student affixed a human nose hair—it had, he said, “just the right length and springiness” for teasing the scales onto new mounts.

The SEM revealed two types of scales. One was solid, characteristic of what were thought to be the oldest Lepidoptera. The other scales had perforations indicating that they were hollow. This trait is common to moths and butterflies of the Glossata, a branch of Lepidoptera characterized by a straw-like feeding mechanism known as the proboscis.

This finding presented a challenge. It was a well-known tenet of evolutionary biology that moths and butterflies with proboscises evolved in conjunction with flowering plants. The proboscis allowed insects to get at the nectar tucked in the throats of flowers, thereby also pollinating the plants. This conjunction, says Strother, was held to be “a classic example of co-evolution.” However, flowering plants do not appear in the fossil record until around 130 million years ago, some 70 million years after Strother’s moths began roaming the earth.

There were two possible explanations. One: Flowering plants had evolved at least 70 million years earlier than was commonly believed, and the fossil record of these plants had yet to be discovered. Darwin had speculated about this possibility. The abrupt appearance of flowering plants, and their rapid diversification roughly 100 million years ago, troubled him. In an 1879 letter to the English botanist Joseph Dalton Hooker, Darwin referred to this truncated process as an “abominable mystery.” He theorized there might have been earlier flowering plants on a lost island or continent. Or Two: Maybe these dramatically older Lepidoptera with developed proboscises started out feeding on something other than flowers.

Strother and his colleagues conferred with specialists who study ancient gymnosperms, the cone-bearing relatives of modern pine trees that date back some 385 million years. Together they concluded that the proboscis was likely used to suck up droplets of fluid secreted by the incipient seeds of gymnosperms. The liquid, called a “pollination drop,” captured airborne pollen grains and facilitated pollination. For the ancient moths, the drop provided sugar-rich nutrition. The scientists speculate that in the hot and arid environment of 200 million years ago, sucking up free moisture via a proboscis was a way to replenish lost body fluids. When flowering plants appeared later, these moths and butterflies were already primed for them.

It took a seven-member team to come up with this new answer to the revived riddle of which came first—the butterfly or the flower, and their findings appeared as “A Triassic-Jurassic Window into the Evolution of Lepidoptera” in the January 2018 issue of Science Advances, published by the American Association for the Advancement of Science. The examination of additional samples is underway, says Strother. “There are still plenty of Mister Greys out there to be studied.”